some question about vector[-NULL]
On 10/09/2014, 3:21 AM, PO SU wrote:
Dear expeRts,
I have some programming questions about NULL in R.There are listed as follows:
1. I find i can't let a list have a element NULL:
a<-list()
a$ress<-1
a$res<-NULL
a
str(a)
You can do it using a <- list(ress = 1, res = NULL)
How can i know i have a named element but it is NULL, not just get a$xxxx,a$iiii,a$oooo there all get NULL
That's a little harder. There are a few ways: "res" %in% names(a) & is.null(a[["res"]]) or identical(a["res"], list(res = NULL)) or is.null(a[[2]]) should all work. Generally because of the special handling needed, it's a bad idea to try to store NULL in a list.
2.The most important thing: a<-1:10 b<-NULL or 1 a<-c(a,b) will work so i don't need to know whether b is null or not,but: a[-NULL] can't work!! i just need a[-NULL]==a , how can i reach this purpose?
Using !, and a logical test, e.g. a[!nullentry(a)] where nullentry() is a function based on one of the tests above, but applied to all entries. Duncan Murdoch